pgsql: Make superuser.c maintain a simple one-entry cache holding the

Log Message:
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Make superuser.c maintain a simple one-entry cache holding the superuser
status of the most recently queried userid.  Since the common pattern is
many successive queries about the same user (ie, the current user) this
can save a lot of syscache probes.

Modified Files:
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    pgsql/src/backend/utils/misc:
        superuser.c (r1.30 -> r1.31)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/superuser.c.diff?r1=1.30&r2=1.31)